Thermosetting powder coatings are environmentally friendly paints with extremely small quantity of solvent emission, and their market has been extended particularly in Europe and the U.S. where the VOC (Volatile Organic Compound) emission is strictly regulated.
The thermosetting powder coatings are broadly classified into four groups of polyester powder coatings, epoxy powder coatings, polyester/epoxy hybrid powder coatings and acrylic powder coatings, and their markets have been developed for various uses according to their prices and performance. Of these powder coatings, the acrylic powder coatings are generally used as decorative coatings assumed to be used outdoors as automotive exterior coatings or the like, making the most of high transparency and out-door-durability (weatherability) inherent in the acrylic resin.
In case of the automotive exterior coatings, the resulting cross-linked cured films are required to have various excellent properties in addition to high gloss. For example, resistance to scratches (marring) caused by car washing brush, sand, or dust, properties of being not etched chemically by acid rain (acid resistance), and properties of being not damaged by organic solvents such as gasoline (solvent resistance) are important. In these typical properties, the acrylic powder coatings are not always superior to the solvent-borne acrylic paints conventionally used. Particularly, improvement of scratch/mar resistance and the chemical properties was a serious problem.
Regarding the improvement of scratch/mar resistance and/or chemical properties of the coating films obtained from the acrylic powder coatings, various studies have been made centering around the thermosetting powder coatings comprising a glycidyl group-containing acrylic resin and a polycarboxyl curative. The reason is that this combination (combination of a glycidyl group-containing acrylic resin and a polycarboxyl curative) is likely to form a smooth and high-gloss coating film and the curing system of glycidyl group/carboxyl group is inherently excellent in the acid resistance.
The approaches, which have heretofore been made for improving scratch/mar resistance and/or chemical properties, are broadly classified into three groups That is to say, the first approach is a method of enhancing the cross-linking density of the coating film by selecting a specific curative, the second approach is a method of enhancing relatively the mechanical strength of the coating film by, for example, adding specific hard particles as additives, and the third approach is a method of imparting special functions, such as slipping function, hyper-hydrophoboic character, to the surface of the coating film by, for example, modifying the glycidyl group-containing acrylic resin.
The method of enhancing cross-linking density of the coating film by selecting a specific curative, which is the first method to improve scratch/mar resistance and/or chemical properties, is described in, for example, EP696622 (publication (A)), and in the publication (A), an aliphatic dibasic acid curative and a linear polyacid anhydride curative obtained by dehydrocondensation of the dibasic acid curative are used in combination. In this publication, acid anhydride groups in a linear polyacid anhydride curative can give new branching points in the thermosetting reaction with glycidyl groups in the glycidyl group-containing resin, whereby enhancement of cross-linking density is achieved. In Japanese Patent Laid-Open Publication No. 137083/1997 (publication (B)), a bi- or trifunctional carboxyl curative obtained by the reaction of TGIC (triglycidyl isocyanurate) with dodecanedioic acid is used to achieve enhancement of cross-linking density.
In the method of enhancing the mechanical strength of the coating film by adding specific hard particles as additives, which is the second method to improve scratch/mar resistance and/or chemical properties, the type of the powder coating is not specifically restricted. For example, in EP853095 (publication (C)), α-alumina fine particles having an average particle diameter of not more than 5.5 microns are added as additives, and in DE19857316, nano-scale ceramic fine particles having an average particle diameter of not more than 100 nanometers are added as additives. In either case, the mechanical strength of the coating film is relatively enhanced.
The third method to improve scratch/mar resistance and/or chemical properties is a method of imparting special functions, such as slipping function, hyper-hydrophoboic character, to the surface of the coating film by, for example, modifying the glycidyl group-containing acrylic resin.
Such a method is described in, for example, Japanese Patent Laid-Open Publication No. 2311894/1996 (publication (D)) and WO9515347 (publication (E)). In these publications, a silicone type macromonomer is copolymerized as an ethylenically unsaturated monomer for constituting the glycidyl group-containing acrylic resin. In EP897962 (publication (F)), glycidyl groups in the glycidyl group-containing acrylic resin are partially modified with a silicone polymer having a functional group.
Although the above methods all contribute to the improvement of scratch/mar resistance and/or chemical properties of the coating film, they still have many problems to be solved in the practical use and are not satisfactory.
Then, the present inventors have earnestly studied a fourth approach different from any of the above methods on the assumption that if the completeness of the thermosetting reaction between a glycidyl group-containing acrylic resin and a polycarboxyl curative is enhanced as highly as possible by improving the mutual solubility or dispersibility of these components for constituting the thermosetting powder coating, a cured film remarkably excellent in the scratch/mar resistance and/or the chemical properties is obtained.
The reasons are described below in detail. Powder coatings are generally prepared by mechanically melt blending a resin component, which is a main binder resin being solid at room temperature, with a curative component, which is also solid at room temperature, and appropriate additives by an extrusion blending machine (extruder) or the like in a molten state and then subjecting the blend to cooling solidification, pulverization and sieving. The meld blending operation is usually carried out under the conditions of such temperature and residence time as bring about substantially no premature thermal curing reaction, and in case of, for example, acrylic powder coatings, the melt blending operation is carried out at a temperature of usually 60 to 130° C. However, it is thought that in this temperature range, a blending or dispersion state up to the molecular level is not reached, because all of the resin component, the curative component, and the additive component are not necessarily melted completely, and the difference of melt viscosity of these raw materials can not necessarily allow the homogeneity in the melt blend. Furthermore, although the dry blending operation with mechanical crashing is generally done in advance to help the homogeneous melt blending of all of the resin component, the curative component, and the additive component, the mechanical crashing of acrylic resin is apt to give low bulk density of the dry blend, which causes too low shear stress in the extrusion blending machines to get homogeneous melt blend due to low packing efficiency. Therefore, the acrylic thermosetting powder coating compositions typically comprising of glycidyl-group containing acrylic resins and polycarboxyl curatives are very difficult to be homogeneous compound or the melt blend. Actually, according to the data of the inventors, scratch/mar resistance and/or chemical properties of the coating film can be generally improved by only heighten the baking temperature. This fact indirectly supports the possibility in the above assumption that the completeness of the thermosetting reaction between a glycidyl group-containing acrylic resin and a polycarboxyl curative can be enhanced by improving the mutual solubility or dispersibility of these components for constituting the thermosetting powder coating.
On the other hand, in case of the solvent-borne acrylic paints, the resin component and the curative component can be easily blended and dispersed to the molecular level by the use of an organic solvent, though there is a problem of regulation of VOC emission. For this reason, the coating film obtained from the acrylic thermosetting powder coating is thought to be inferior to the coating film obtained from the acrylic thermosetting solvent-borne paint in the film properties such as scratch/mar resistance and/or chemical properties.
An approach to improve the blending or dispersion state of the resin component and the curative component from the viewpoint of thermosetting powder coating production process is, for example, a method comprising completely dissolving the components by the use of tertiary butanol capable of dissolving both of the resin component and the curative component and then removing the tertiary butanol by freeze drying, as described in GB2326883 (publication (G)). Further, in U.S. Pat. No. 6,114,414 (publication (H)) and WO9534606 (publication (I)), an inert fluid in a supercritical state is used as a solvent medium for promoting blending and dispersing. In Japanese Patent Laid-Open Publication No. 192604/2001 (publication (J)), a method comprising melt blending the components in the presence of an organic solvent and recovering the solvent simultaneously with the melt blending by means of a pressure reducing device equipped in an extruder is proposed. These techniques can be all expected to exert great effects, but they are accompanied by changes of the production process or apparatuses, so that it is hardly to say that they are practically used at present.
Under such circumstances as mentioned above, the present inventors have further studied earnestly, and as a result, they have found that the blending efficiency and dispersibility of the components, particularly those between the glycidyl group-containing acrylic resin and the polycarboxyl curative, can be remarkably improved by the use an alcohol adduct of a styrene/maleic anhydride copolymer as a dispersion promoter in the acrylic powder coatings comprising the glycidyl group-containing acrylic resin and the polycarboxyl curative, without changing the conventional production process of a powder coating, and a cross-linked cured film obtained from the composition can be remarkably improved in appearance properties (high gloss, etc.), physical properties (hardness, scratch/mar resistance, etc.) and chemical properties (acid resistance, solvent resistance, etc.), particularly in scratch/mar resistance and chemical properties. Based on the finding, the present invention has been accomplished.